This invention is related generally to labyrinth sealing apparatus of the type used in steam turbines to minimize leakage of steam between the regions of differential pressure through which a rotor of the turbine extends, and more particularly to methods and apparatus for minimizing the effects of steam whirl in such turbines through the addition of flow dams in such sealing apparatus.
As is well known, non-contacting packing ring labyrinth seals are conventionally used in steam turbines at selected axial positions along the length of the turbine rotor to minimize steam leakage between regions of differential pressure. Such packing ring labyrinth seals typically include a plurality of spaced-apart annular teeth, known as seal strips, which extend radially inward from the turbine casing suspended from seal carriers. The distal end of each such seal strip is disposed in close proximity to the rotating surface of the rotor, leaving only a very small clearance therebetween, in order to provide a minimum-leakage seal. These types of seals are very effective, and are utilized not only to prevent steam from leaking out around the turbine shaft, but are also utilized to prevent leakage of steam between stages of the turbine where the shaft passes through the diaphragms.
As is also well known, a certain amount of steam continuously enters and exits the packing ring structure with a flow component generally along the shaft in an axial direction. However, the steam flow entering and exiting the packing ring structure also has a component, typically referred to as "swirl", in the circumferential direction.
It is a generally accepted notion that swirl results from two principal causes: (1) a whirling or circumferential component imparted by the most adjacent upstream turbine stage; and, (2) a circumferential flow component produced by the drag effect of the rotating shaft. Although the latter such component is always in the direction of rotor rotation, the former such whirling component may be in either direction depending upon the operating parameters of the most adjacent upstream turbine stage. For example, it is known that the turbine stage that supplies steam to the end packing seals in turbines with double flow first stages produces a forward running swirl (i.e., in the direction of shaft rotation) at high loads.
At low loads, or in cases where the rotor is operating at less than full-speed, tests have shown that steam flowing circumferentially in the space above the rotor between the seal legs has a highly destabilizing effect on the rotor whirl when such circumferential flow is in the same direction as the direction of rotor rotation. On the other hand, steam flow in the same seal space opposite to rotor rotation has a stabilizing effect on rotor whirl. As is readily apparent, therefore, it is primarily rotational instabilities caused by swirl which lead to the above described destabilizing effect on rotor whirl.
Steam flow within the seal structure is known to produce lateral forces on the turbine rotor due to asymmetrical pressure gradients which arise in the seal chambers. In some cases where it is known that forward swirl within the shaft end seals is strong, the turbine rotor begins to experience rotational instability related to the steam whirl conditions. In particular, such as in turbines of the double flow type discussed herein above, there is a susceptibility to such rotational instabilities at higher load levels associated with forward swirl within the seals. More often than not, however, such rotational instabilities cannot be discovered until the installation of the turbine is complete and a full load cannot be achieved due to the levels of vibration which are usually experienced when attempting to reach full load under conditions conducive to the generation of swirl. It would, therefore, be desirable to provide a method and apparatus for minimizing steam whirl in turbines which may be installed in the field as a retrofit to existing apparatus. In such a manner, downtime for installation of the apparatus would be minimized while at the same time further downtime for diagnosis and repair of problems related to rotor instabilities brought about by swirl would be prevented.
One attempt to deal with the above described problem, although not necessarily from a retrofit point of view, is disclosed in U.S. Pat. No. 4,273,510, issued June 16, 1981 to Ambrosch et al. In that patent, Ambrosch et al. disclose a method of and devices for avoiding rotor instability to increase the dynamic power limit of rotary fluid machines such as turbines and compressors which have contact-free seals in the gaps between the rotating and nonrotating elements of the machine in which the rotary flow of fluid in the gap is modified either by axial baffles in the gap or by introducing a fluid medium into the gap thereby to decrease, eliminate, or even reverse the force component acting on the rotating element and disposed 90.degree. ahead of the oscillation deflection of the rotating element. One problem with such an arrangement, however, is that it is complex and not readily adaptable to be retrofitted within an installed turbine. In particular, where the method and devices of Ambrosch et al. require the introduction of another fluid medium into the gap to decrease, eliminate, or even reverse the force component acting on the rotating element, implementation of such secondary flow to function properly would not be without difficulty.
Another prior art approach to the stabilization of turbine rotors subjected to steam whirl through modifications to the labyrinth seals of such rotors is disclosed in U.S. Pat. No. 4,420,161, issued Dec. 13, 1983 to Miller. Miller teaches the use of a fixed circumferential row of spaced-apart flow directing vanes encircling the rotor on the upstream side of a plurality of fixed, spaced-apart annular teeth which surround the shaft of the turbine whereby each tooth has a radially inner edge in very close proximity to the rotor surface. Each vane of the row extends radially inward to within very close proximity of a raised annular land on the shaft surface just opposite the vane row. Miller further teaches that the row of flow directing vanes and the raised land cause substantially the entire quantity of steam which enters the seal to pass through the row of flow directing vanes, whereby steam flow within the seal is caused to have a retrograde component counter to the direction of shaft rotation which produces stabilizing forces on the rotor to neutralize destabilizing forces caused by swirl.
An apparatus utilizing such flow directing or "preswirl" vanes, however, would suffer from two primary disadvantages. First of all, the preswirl obtained by the flow directing vanes would be easily overcome by the viscous drag of the rotor on the fluid as the steam passes through the narrow gap between the rotor and seals. In addition, the velocity of the steam passing through the preswirl vanes would be necessarily small because the labyrinth seal is designed to minimize axial flow by use of small radial clearances between the seal leg and rotor, and because the radial height of the preswirl vanes would be large by comparison. Accordingly, it would be more desirable to provide a method and apparatus which would reduce circumferential fluid flow in the cavities between the seal legs to a very small quantity.
With such goal in mind, U.S. Pat. No. 4,370,094, issued Jan. 23, 1983 to Ambrosch et al. discloses flow guiding structural components in the form of deviating plates, baffles, fins, profiles, passages, or the like which are arranged in the gap region in front of, or in the seals. The flow guiding structural elements, according to Ambrosch et al., are arranged so that the mean circumferential component of the gap flow is so decreased that the force component of the pressure distribution which runs ahead of the oscillation deflection of the rotor oscillation by 90.degree. is reduced, eliminated, or reversed in its direction. In particular, flow guiding structural elements 6a (as shown in FIG. 4 of Ambrosch et al.) each of which have an inward radial extent within the gap slightly less than the radial extent of the seal legs are disposed between the seal legs, ostensibly to reduce differentials in fluid pressure around the gap thereby resulting in a reduction of self-excited oscillations of the rotor and allowing an increase in the power imparted to the blades of the turbine. However, the flow guiding structural elements 6a as taught in Ambrosch et al. still suffer from an inability to adequately reduce the circumferential flow known as swirl since they do not sufficiently cover the cavities between the seal legs. It would, therefore, be more desirable to provide a method and apparatus for stabilizing steam turbine rotors which substantially covers the cavities between the seal legs, thereby minimizing circumferential fluid flow causing swirl.